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This addresses the current crashing bug that takes place whenever popup windows
are spawned, due to the fact that no WindowParentingClient exists to handle
the newly spawned ui::Window. With this solution, the child windows are
never spawned in the first place.

* Relocated 'content' overrides to FrameImpl's private declarations.
* Misc. IWYU cleanups.

Bug: 888131
Change-Id: I735565047bbe4c8ebab5154a57ad323f33ef8e48
Reviewed-on: https://chromium-review.googlesource.com/1239419
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593402}